Conventionally, as a negative-resistance device, there have been proposed various super lattice elements including those formed from single crystals of inorganic materials such as GaAs, AlAs and the like, and those formed from a combination of a single crystal of inorganic material and an inorganic amorphous material or electrically insulating organic polymer. Most of these are such that a thin film is formed by the molecular beam epitaxy technique (hereinafter referred to as an MBE process). This requires treatment under extremely severe conditions, such as ultra-high vacuum, high temperature conditions and the like and the use of a very costly and large-sized film forming apparatus. The process also requires strict control, and this makes it difficult to manufacture such a device uniform over a wide area. Further, semiconductors available for use are limited.
In the Japanese Patent Application Laid-Open Publication No. 6-29556, there is proposed a resonant tunneling diode having a negative-resistance characteristic which includes a fullerene thin film formed from a carbon cluster as an organic material and an electrically insulating thin film in which is used a metal oxide, the thin films being sandwiched between electrodes. This publication points out or suggests that an electrically insulating organic compound and an electrically insulating organic polymer matrix can be used as an electrically insulating layer. However, it is difficult to form an electrically insulating layer to the thickness of 3 to 15 nm, a thickness required for obtaining negative-resistance characteristics. In the publication, no example is shown of a tunnel diode in which the electrically insulating layer is formed from such electrically insulating organic materials.
Recently, an electroluminescence device having a quantum well structure formed by vacuum vapor depositing ultra-thin films of organic dyes alternately by an organic molecular beam evaporation method has been proposed in, for example, the prior art literature 1, Hiroshi Ohmori, et al, "Organic Quantum Well Structure EL Device", Monthly publication of The Japan Society of Applied Physics, Vol. 64, No. 3, pp. 246-249, 1995. However, the device has not demonstrated any negative-resistance characteristic.
In an operation at a room temperature, a negative-resistance device is required to exhibit a large ratio of peak current to valley current (the term "ratio of peak current to valley current" is hereinafter referred to as a "PV ratio"). However, the PV ratio of a resonant tunnel diode in which GaAs is used is about 7.7 at a temperature of 77 K (See, for example, prior art literature 2, Takayuki Nakagawa, et al, "Effect of Prewell Insertion in InGaAs/A1As/InAs resonant tunneling diodes", Papers at Autumn General Meeting, 1995 of the Japan Society of Applied Physics Society, 29a-Zm-8, 1995).
On a junction interface of a negative-resistance device using an inorganic single crystal there exists a non-joinable surface level arising from dangling bonds which is characteristic of crystals. Development of such a surface level and degradation of crystallinity on the junction interface leads to lowering the interface electric field, thus lowering the efficiency of carrier injection with the result that the resonant-tunneling effect of the device is substantially affected.
A device comprising a combination of an electrically insulating organic polymer and an inorganic single crystal is inevitably thermally unstable and physically fragile because the thermal expansion coefficient of organic material is larger than that of inorganic material by about an order of magnitude. Inorganic single crystal materials are only available in limited kinds, i.e., GaAs and Si, and therefore negative-resistance devices using such materials permit only a lower freedom of material selection. Further, the prior art layer structure which comprises alternately stacked crystals of different compositions is complex in construction and is less processable. The MBE process which has hitherto been largely employed upon manufacturing the negative-resistance devices is such that crystal growth of component elements is carried out in ultra-high vacuum and under strict control. This poses the problem of lower productivity. Another problem is that the process requires a very costly film forming apparatus.
An object of the present invention is to provide a negative-resistance device which solves the above-mentioned problems, is easier to manufacture, can be manufactured at lower cost as compared with the prior art device, and yet has a comparatively large PV ratio, and a method for manufacturing the device.